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Research Papers: Flows in Complex Systems

Vortex Rope Formation in a High Head Model Francis Turbine

[+] Author and Article Information
Rahul Goyal

Department of Mechanical
and Industrial Engineering,
Indian Institute of Technology,
Roorkee 247667, India;
Division of Fluid and Experimental Mechanics,
Department of Engineering Sciences
and Mathematics,
Lulea University of Technology,
Norrbotten 97187, Sweden
e-mail: goel.rahul87@gmail.com

Michel J. Cervantes

Professor
Division of Fluid and Experimental Mechanics,
Department of Engineering Sciences
and Mathematics,
Lulea University of Technology,
Norrbotten 97187, Sweden;
Water Power Laboratory,
Department of Energy
and Process Engineering,
Norwegian University of Science
and Technology,
Trondheim 7491, Norway
e-mail: Michel.Cervantes@ltu.se

B. K. Gandhi

Professor
Mem. ASME
Department of Mechanical
and Industrial Engineering,
Indian Institute of Technology,
Roorkee 247667, India
e-mail: bkgmefme@iitr.ernet.in

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received June 16, 2016; final manuscript received November 7, 2016; published online February 16, 2017. Assoc. Editor: Riccardo Mereu.

J. Fluids Eng 139(4), 041102 (Feb 16, 2017) (14 pages) Paper No: FE-16-1370; doi: 10.1115/1.4035224 History: Received June 16, 2016; Revised November 07, 2016

Francis turbine working at off-design operating condition experiences high swirling flow at the runner outlet. In the present study, a high head model Francis turbine was experimentally investigated during load rejection, i.e., best efficiency point (BEP) to part load (PL), to detect the physical mechanism that lies in the formation of vortex rope. For that, a complete measurement system of dynamic pressure, head, flow, guide vanes (GVs) angular position, and runner shaft torque was setup with corresponding sensors at selected locations of the turbine. The measurements were synchronized with the two-dimensional (2D) particle image velocimetry (PIV) measurements of the draft tube. The study comprised an efficiency measurement and maximum hydraulic efficiency of 92.4 ± 0.15% was observed at BEP condition of turbine. The severe pressure fluctuations corresponding to rotor–stator interaction (RSI), standing waves, and rotating vortex rope (RVR) have been observed in the draft tube and vaneless space of the turbine. Moreover, RVR in the draft tube has been decomposed into two different modes; rotating and plunging modes. The time of occurrence of both modes was investigated in pressure and velocity data and results showed that the plunging mode appears 0.8 s before the rotating mode. In the vaneless space, the plunging mode was captured before it appears in the draft tube. The physical mechanism behind the vortex rope formation was analyzed from the instantaneous PIV velocity vector field. The development of stagnation region at the draft tube center and high axial velocity gradients along the draft tube centerline could possibly cause the formation of vortex rope.

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Figures

Grahic Jump Location
Fig. 1

Sensor placement as seen from the top (a) and from the side (b). All dimensions are in millimeter. The sensor corresponding to the numbers are shown in Table 1.

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Fig. 2

Line diagram to represent synchronization between pressure and PIV measurements

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Fig. 3

Raw (a) and smoothed (b) velocity vector field

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Fig. 4

Signals and corresponding mean obtained during load rejection (BEP to PL) for 20 repetitions, (a) GV angle (α), (b) head (H), (c) discharge (Q), and (d) generator torque (TGEN). Gray dot lines: signals from 20 repetitions; solid line: mean signal of the 20 repetitions; black dashed lines: start and end of the transient process.

Grahic Jump Location
Fig. 5

Time-dependent variation in the GV angle (α), head (H), discharge (Q), and generator torque (TGEN) for the measurement investigated

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Fig. 6

Mean axial and radial velocities for 20 repetitions. In each graph, the notation at the left stands for the operating point and at the right for the measurement line.

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Fig. 7

Absolute flow velocity (V) in the draft tube at (a) BEP and (b) PL operation

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Fig. 8

Spectrogram of the pressure sensor located in the vaneless space (VL1) during load rejection from BEP to PL. Black solid line: GVs angle (α) with the y-scale to the right.

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Fig. 9

Spectrogram of the pressure sensor (DT2) located in the draft tube during load rejection from BEP to PL. Black solid line: GVs angle (α) with the y-scale to the right.

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Fig. 10

Spectrogram (low frequency) of the pressure sensorlocated in the draft tube during load rejection (BEP to PL) at DT2. Black solid line: GVs angle (α) with the y-scale to the right.

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Fig. 11

Spectrogram, low frequency region, of the velocity signal at P1 during load rejection BEP to PL: (a) radial velocity (u) and (b) axial velocity (v). Black solid line: GVs angle (α) with the y-scale to the right.

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Fig. 12

Time of appearance of RVR frequency in the vaneless space sensors VL1 and VL2 during load rejection, GVs angle (α) is with the y-scale to the right

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Fig. 13

Spectrogram of the RVR rotating and plunging modes for the pressure sensor (DT2 and DT4) located in draft tube during load rejection (BEP to PL): (a) plunging mode and (b) rotating mode. Black solid line: GVs angle (α) with the y-scale to the right.

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Fig. 14

Appearance of the rotating and plunging modes for the pressure sensors DT2 and DT4 during load rejection from BEP to PL, GVs angle (α) is with the y-scale to the right, black solid line: appearance of plunging, black dashed line: appearance of RVR

Grahic Jump Location
Fig. 15

Appearance of RVR rotating and plunging modes for the signal of the velocity at points P1 and P2 in the draft tube during load rejection, BEP to PL: (a) radial velocity (u*), (b) axial velocity (v*), GVs angle (α) is with the y-scale to the right, black solid line: appearance of plunging, black dashed line: appearance of RVR

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Fig. 16

Instantaneous normalized absolute velocity (V*) at line 1 during load rejection. Black solid line is starting of transient operation and black dashed line is ending of transient operation.

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Fig. 17

Instantaneous normalized axial and radial velocities at line 1. Black solid line: starting of transient operation and black dashed line: ending of transient operation.

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Fig. 18

Instantaneous normalized radial velocity (u*) at line1, black solid lines: starting of the transient operation. Black solid line: starting of transient operation and black dashed line: ending of transient operation.

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Fig. 19

Gradient of axial velocity in the draft tube during load rejection, t = 0 s at BEP, t = 2.5 s at PL: (a) t = 0 s, (b) t = 0.5 s, (c) t = 1.5 s, (d) t = 2.0 s, (e) t = 2.5 s, and (f) t = 3.0 s

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Fig. 20

Instantaneous velocity vector field during load rejection, t = 0 s at BEP, t = 2.5 s at PL: (a) t = 0 s, (b) t = 0.5 s, (c) t = 1.5 s, (d) t = 2.0 s, (e) t = 2.5 s, and (f) t = 3.0 s

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